Running Arduino for an extended period of time

So I'm using the arduino (diecimila) for a light I am making, and like most lights, it will go long periods of being off or on with no user input and the same output from the arduino. (The code is essentially the same as the blinking LED bike light from the LadyAda tutorials - a group of LED's with a few different light modes controlled by a single button)

My question - how does the arduino hold up over long periods of time? If the light sits in an "off" state for a week (with the arduino waiting... ardently... for button input) will the chip start to overheat? Will it go to sleep? Will it go insane? What if it is "on"? (In which case the arduino will be using all six PWM pins to hold six corresponding transistors at saturation voltage)

The chip isn't going to be in an especially tight enclosed space - something the size of a shoebox.

Any sort of advice/experience would be appreciated.

This is an interesting issue. I too am working on a project that requires Arduino running unattended for weeks if not months... Can anyone share his/her experiences with their long duration tests... how stable is Arduino over the long run...?

It'll be fine. You'll find similar chips in every appliance in your house. :P

There may be millis() and/or delay() glitches. They are easy to work around though if the glitches are still there.